1. Technical Field
The present invention relates to a vehicle travel control device for performing travel control by recognizing a travel lane of a host vehicle and a preceding vehicle traveling ahead of the host vehicle.
2. Related Art
In the related art, in a vehicle such as an automobile, following travel control is known in which a camera or a radar detects a travel lane of a host vehicle or a preceding vehicle traveling ahead of the host vehicle so as to control an inter-vehicle distance from the preceding vehicle to be a suitable distance, and in which a lateral position of the host vehicle inside the travel lane is controlled so that the host vehicle follows a lane center position or a center position of the preceding vehicle.
For example, Japanese Unexamined Patent Application Publication (JP-A) No. 2000-20896 discloses a technique in which travel control is performed so as to follow a lane center line when a lane line on a road can be detected, and in which travel control is performed so as to follow a center position of the preceding vehicle when the lane line is hidden by the preceding vehicle and cannot be detected. According to this technique in the related art, when the host vehicle travels to follow the preceding vehicle, a control gain is weakened in a case where a possibility that the preceding vehicle may turn to the right or left is determined, based on map information, and following control is weakened by estimating zigzag travel of the preceding vehicle, based on a change in lateral displacement of the preceding vehicle, thereby attempting to reduce an improper behavior change of the host vehicle.
However, the technique disclosed in JP-A No. 2000-20896 gives no consideration to controllability required when the host vehicle switches between the travel to follow the lane and the travel to follow the preceding vehicle. Under a condition that the preceding vehicle travels biased from a lane center line, there is a possibility that a behavior change of the host vehicle may greatly increase when the travel control to follow the lane is switched to the travel control to follow the preceding vehicle.
For example, as illustrated in FIG. 5, in a three-lane road, a center lane has broken line-shaped lane lines L1 and L2 on both sides. Accordingly, when a host vehicle C travels along the center lane, the host vehicle C periodically meets a scene in which the lane lines L1 and L2 on both sides are out of a camera's view R at the same time and are not recognizable. In a state where the lane lines L1 and L2 on both sides are recognized, the host vehicle C travels to follow a center position of the lane lines L1 and L2. However, when the lane lines L1 and L2 on both sides are not recognizable, the travel control is switched to the travel control to follow a preceding vehicle C1. In this case, if a center position of the preceding vehicle C1 is offset from the lane center position, a target point in the following travel is changed from the lane center position to the center position of the preceding vehicle C1, thereby causing the host vehicle C to travel unsteadily. Furthermore, there is a possibility that the host vehicle C may travel zigzag due to periodically switched following target points.